The use of conventional electromechanical bell ringers has been the primary signaling device used in telephones for a great number of years. In recent years, however tone ringers considered to be more desirable have been replacing the electromechanical bell ringer due to advances in electronic technology. The tone ringers provide a signal generally considered substantially more pleasing to the average ear than the sound of an electromechanical bell ringer. Also, space requirements through use of tone ringers are minimized. These tone ringers comprise both electronic circuitry that responds to conventional low frequency power ringing signals on a telephone line, and also a tone generating transducer that provides an alerting signal to a telephone subscriber. An example of such a tone ringer is disclosed in U.S. Pat. No. 4,276,448, issued to M. L. Embree et al. on June 30, 1981.
Telephone ringers have to reliably operate in an environment containing a wide variety of other signals that are intentionally and unintentionally coupled to the telephone line. Some electronic ringers in the art have had the tendency to sound in response to these signal transients on the telephone line caused by dialing pulses or the line being seized prior to ringing. Because of the magnetic and mechanical time delays inherent in the electromechanical ringer, it was not prone to respond to such line transients. It is desirable, therefore, to provide a tone ringer with these general characteristics which is capable of increased accuracy in distinguishing between valid ringing signals and the voltage pulses and other transients coupled to the telephone line.
Telephone ringers have to also operate over a loop whose length can vary considerably. Although more efficient than electromechanical bell ringers, tone ringers of the known art are optimized to operate over loops not exceeding a certain length since the ringers have an operational voltage range. Telephone extensions with ringers increase the load presented to the ringing voltage resulting in a further decrease in the available voltage. Thus, the value of the current available to the tone ringer is a critical, sensitive factor, particularly so in those instances where multiple sets are terminated on a line. It is desirable, therefore, to provide a tone ringer which allows for efficient operation over a wide range of loop lengths and multiple extensions being terminated on the telephone line.
Since telephone ringers are usually terminating a telephone line at all times, they should have an input impedance characteristic which is tailored to be very high for low voltage levels in the voice frequency band so as to not attenuate voice signals. Furthermore, it is desirable to have the telephone ringer provide a unique impedance signature so that the continuity of the telephone loop from the central office to the telephone subscriber can be verified by loop maintenance test circuits located at the central office. This capability could aid in determining whether an existing trouble condition is on the telephone loop or at the subscriber's premises. It is desirable, therefore, to provide a tone ringer with a controllable input impedance characteristic.
In addition, tone ringers, in order to be truly effective, ought to have the capability to produce distinctive audible signals. The ever increasing range of services offered to telephone subscribers including PICTUREPHONE.RTM. Meeting Service, audio teleconferencing, intercom calling, as well as conventional telephone service, necessitates the development of means to distinguish between the sound produced by one telephone or type of service and the audible signal produced by other sets or services. Where conventional electromechanical bell ringers are used, it is difficult to provide user controllable distinctive ringing, since their sound characteristics are usually varied by adjusting the natural frequency of the resonating element. On the other hand, where tone generating tranducers are used, electronic circuitry can be provided to simply achieve the same result. Accordingly it is desirable to provide a tone ringer with an easily implemented distinctive ringing capability.
For low cost telephones which do not need a distinctive ringing capability, a piezoelectric transducer is often used. This type of transducer is characterized in that it has a very narrow frequency band width of peak efficiency. Accordingly, it is desirable for the tone ringer to produce a driving signal with a precisely controlled frequency in order to consistently drive a piezoelectric transducer at the frequency of its peak efficiency.